.box{
    width: 100vw;
    height: 100vh;
    background: pink;
}
.form{
    position: fixed;
    width: 300px;
    height: 200px;
    background: #fff;
    left: 500px;
    top: 200px;
}
.form Form{
    margin-left: 30px;
}
.form .btn{
    width: 200px;
    position: fixed;
    right: 780px;
}